1. Field of the Invention
This invention relates to a process for preparing glass fiber mats.
More particularly, this invention relates to a process for preparing glass fiber mats having a low solution velocity in a vinyl monomer.
2. Description of the Prior Art
It is common to use glass fiber mats as reinforcing agents for fiber reinforced plastics (FRP). Unsaturated polyester resin powders have been used as binders for the glass fiber mats. The glass fiber mats are prepared on an industrial basis by sprinkling the unsaturated polyester resin powder onto the glass fibers and placing the assemblage in a furnace to melt the resin, whereby the molten resin is bonded firmly to the glass fibers. Depending on the method of forming the FRP products, glass fiber mats having various solution velocities in vinyl monomers are demanded.
For obtaining a glass fiber mat having a low solution velocity in vinyl monomers, an unsaturated polyester resin having a low solution velocity in vinyl monomers should be used. However, many of the known unsaturated polyester resins have relatively high solution velocities in vinyl monomers and, accordingly, they do not exhibit a satisfactory property in this regard. A conventional method of obtaining a glass fiber mat having a low solution velocity in vinyl monomers comprises adding 1 to 3 weight percent of a catalyst such as benzoyl peroxide to the resin and heating the same in a furnace to cause a cross linking reaction of the resin. However, this method has a disadvantage that a satisfactorily uniform cross linking reaction does not always take place, because the cross linking reaction of the unsaturated polyester resin is caused by fumarate double bonds of the molecules and the benzoyl peroxide added as catalyst is in the form of a powder. Another disadvantage owing to the use of benzoyl peroxide is that the use of benzoyl peroxide is attended with a danger of explosion. Particularly, when benzoyl peroxide in the form of a powder is mixed with the resin powder, the operation is quite dangerous.
Another method of decreasing the rate of dissolution of a glass fiber met in vinyl monomers comprises controlling the chemical composition of the unsaturated polyester resin used as the binder. By this method, however, the decrease in the rate of dissolution is still insufficient. Generally, a glass fiber mat having a high rate of dissolution in a vinyl monomer is used in a low pressure molding process such as the hand lay-up process. In a low pressure operation, it is permissible that the resin binder is such as to dissolve rapidly in the vinyl monomer contained in the matrix resin to be molded, because the glass fiber mat does not move during the bubble removing operation wherein there is employed a roller under a low pressure. Rapid solution of the resin binder is rather preferred in this instance, because owing to its rapid solution in the vinyl monomer, the glass fiber mat becomes capable of conforming to the shape of the mold more rapidly and easily.
Recently, compression molding methods such as the matched die molding method have been developed and, in such methods, glass fibers mats having a quite low solution velocity in vinyl monomers are needed. In the compression molding operation, glass fiber mats should not dissolve rapidly in the vinyl monomer. Thus, it is preferred to use binders having low solution velocities in the vinly monomer.